The present invention relates generally to methods and apparatus for electrolytic reduction of metal and more particularly to improved methods and apparatus for the continuous provision of a high purity and highly conductive, particulate, free-flowing carbon material to serve as the anode of the reduction cell.
In the prior art, metals are reduced from metallic compounds by means of the fused salt electrolysis cell. This technique is particularly applicable to the reduction of aluminum wherein alumina (Al.sub.2 O.sub.3) is reduced to aluminum metal with the utilization of a carbon anode. The electro-chemical reaction which results in the formation of metallic aluminum yields oxygen at the anode. Oxygen in turn reacts with the anode carbon to form carbon dioxide. The overall electrolysis of alumina can be summarized by the simplified equation: EQU 2Al.sub.2 O.sub.3 +3C.fwdarw.4Al+3CO.sub.2
The theoretical carbon requirement based on this stoichiometry is 0.33 lbs. carbon/lb. aluminum. However, present industry practice requires approximately 0.5 lbs. carbon/lb. aluminum.
Expressed as a quantitative total, the usage of carbon anodes for aluminum production is approaching 5 million tons per year on a worldwide basis. This commercial process is however not without difficulty and improvement therein is indicated to lower costs. In particular, anode carbon is an expensive reagent for a chemical process. Anode carbon is in itself a product of manufacture that must meet close specifications. The carbon anodes which are typically used for this process must have suitable density, low sulfur content and grindability.
One method of producing carbon anodes is by pre-baking of ground carbon. The manufacture of pre-baked bulk carbon anodes is a complex series of operations involving mixing of calcined petroleum coke or anthracite with pitch materials and binders, extruding the mix into the desired shape, and then slow baking in a furnace at temperatures in excess of 1000.degree. C. Alternatively, calcined petroleum coke or calcined anthracite can be mixed as a paste and baked in place as it is used in the electrolytic cell. This continuously formed electrode is known as the Soderberg-type electrode. However, the present trend, because of emission control and process control considerations, is to use the pre-baked carbon shapes.
Means to reduce the consumption of anode carbon during aluminum cell operation is a subject of continuing study by the aluminum producers. Anode loss is not due simply to chemical reaction. It has been found that the carbon residue from the pitch binder is more reactive with oxygen than are the coke particles of the electrode mix, and this selective oxidation of binder coke causes the anode to disintegrate at the working surface. Results of studies suggest that selective oxidation and disintegration is the principal reason for the substantially higher consumption of the anode than that corresponding to the formation of carbon dioxide by the presented chemical reaction.
One possible means to eliminate the consumption of the carbon anode is to use some other electrode material which is inert in use in the electrolysis cell. However, there are very limited possibilities of development of materials other than carbon and graphite that can withstand the highly corrosive molten cryolite, and liberated oxygen and fluorine. Moreover, such non-consumable anode materials must be compatible electrically and thermally with the Hall-Heroult cell requirements.
The ability to use a non-consumable anode is a major incentive for the current interest in development of the aluminum chloride electrolysis route to aluminum metal. In this process, the chloride rather than the oxide is electrolyzed to obtain metallic aluminum. In addition to a number of advantages claimed for the chloride process, graphite electrodes can be used with little consumption because chlorine liberated by the cell reaction is not nearly as reactive with the carbon as is oxygen. The chloride process, however, does require a purified aluminum chloride, which at present is being made by chlorination of alumina. The advantages gained at the electrolysis step are at the expense of the additional and complex processing of alumina into high purity aluminum chloride. Further improvement in electrolytic reduction techniques is indicated.
Anodes made of graphite can be used in the Hall-Heroult cell in place of the pre-baked or Soderberg-type carbon electrodes. Graphite is less reactive and, therefore, consumption of graphite anodes would be substantially lower than that of carbon. This potential advantage of graphite is offset by the fact that graphite is more costly to produce. Substantially higher furnace temperatures and longer baking times are required to manufacture graphite than to make carbon electrodes. Additionally, the lower electrical resistance and higher thermal conductivity of graphite electrodes results in higher heat loss through the electrode column, which leads to a higher rate of oxidation of the electrode at the top. Attempts have been made to incorporate particulate graphite as part of the carbon mix going into the manufacture of such anodes. It is found, however, that it is difficult to form the desired electrode shape using particulate graphite, and other economic factors have precluded the use of particulate graphite for this purpose.
Wherefore, in view of the shortcomings and deficiencies of the prior art, it is a primary objective of the method of the present invention to improve these methods of production of metals and especially aluminum metal by the electrolytic reduction of metallic compounds, such as for example alumina and/or the aluminum halides in alternative embodiments. The improved method and apparatus of the present invention are intended to reduce significantly the consumption of carbon per unit of aluminum produced. It is a further objective hereof to simplify the operation of the aluminum smelter by eliminating the need to periodically replace partially consumed pre-baked electrodes or to continuously produce within the smelter shop the paste electrodes of the Soderberg-type. Further objectives and advantages of the improved method and apparatus of the present invention will be evident from the following brief description of the drawing, detailed description of preferred embodiments and appended claims.